Historically, metal forming has been largely accomplished through strictly mechanical means. As one example, the art of riveting still commonly includes the use of a pneumatic riveting gun, typically in combination with a bucking bar positioned on the opposite side of the workpiece.
Multiple blows of the conventional vibrating riveting gun are used to provide the desired rivet upset. Observation and control by a highly skilled operator is necessary to produce a high quality rivet. Typically the relatively long upset time, i.e. 1-5 seconds, prevents destructive heat build-up in the rivet, and the use of a passive bucking bar is appropriate because of the relatively low momentum of the gun slug.
The conventional riveting gun, however, has the disadvantage of being extremely noisy and its use creates a stressful work place. Also, the requirement of a highly skilled operator, who must control the operation of the gun to produce the desired result, is a disadvantage. Carelessness or lack of attention on the part of the operator can cause expensive damage to workpieces such as airplanes and the like.
Alternatively, a one-shot pneumatic riveting gun is known which has a faster force rise time than the conventional gun. Such a system reduces the need for a highly skilled operator and reduces the overall noise level. However, the force rise time in the oneshot gun is so fast, on the order of 300 microseconds for a system using a 3/4 lb. driver, that stress cracks will result in many types of rivets, which is unacceptable. As an example, the 7050 aluminum rivet will typically develop stress cracks if the force rise time is less than 0.5 millisecond. The driver mass could possibly be increased, but because a passive bucking bar is used with the gun, additional problems result. Typically, if the head of the rivet is hit, the plate will move significantly, while if the tail of the rivet is hit, the head of the rivet is pushed out of the hole, both of which are undesirable.
A somewhat related embodiment is known as a C-yoke squeezer which is a large, expensive device which extends around the workpiece to provide an integral backing member. However, such devices are impractical for many applications, since throat depth requirements, i.e. the distance of the rivet from the edge of the workpiece, result in an apparatus which is impractically large and expensive because of the corresponding stiffness demanded for the required throat depth.
Thus, all of the above mechanical devices have significant disadvantages. Electromagnetic techniques were developed to form metal without a mechanical impact. Initially, electromagnetic fields were used to directly form thin sheet metal, such as exemplified in the U.S. Pat. No. 2,976,907 to Harvey et al. Harvey teaches that the shape of very thin conductive sheet metal could be altered to a desirable configuration by placing the sheet metal in close proximity to a spirally wound pancake coil. Discharging a high voltage through the coil resulted in a rapidly rising magnetic flux, which in turn induced eddy currents into the sheet metal. The repulsive force resulting from the magnetic flux was substantial enough to form the thin sheet metal into a desired configuration.
Since the sheet metal was extremely thin, the current pulse through the coil had to have an extremely fast rise time in order to produce the required metal forming effect . Such a system required a very high voltage, on the order of 10,000 volts, and corresponding sophisticated and expensive high voltage switching apparatus and other circuitry. The entire electrical and mechanical system had to be designed to handle such high voltage.
The next significant development in electromagnetic metal forming technology is exemplified by Patent No. 3,453,463 to Wilde, which basically added a driver element to the electromagnetic metal forming system of Harvey. This resulted in a general purpose actuator which had many more potential applications than just forming sheet metal. In the Wilde system, a thin layer of copper cladding was added to the base of the driver, with the driver configured to include a long nose section extending outward from the unit. The free end of the nose section was the actuator. Riveting dies or similar elements could be attached to the end of the nose section. This system was, and has continued to be, characterized by a fast current risetime and high voltage, like the Harvey system.
When the Wilde system is used for riveting, the advantages include a significant decrease in noise relative to the pneumatic gun, and the accomplishment of the rivet upset (rivet formation) with just one blow instead of multiple blows. Further, the Wilde electromagnetic system does not require a highly skilled operator who must exercise a significant amount of judgment during rivet formation. Also, because of the narrow force pulse produced by the system, the recoil forces in the system are minimal.
The Wilde system was considered to be a significant advance in the riveting art, and similar systems are in fact currently used, in various configurations, although the pneumatic and C-yoke riveters are still widely used, even with the disadvantages noted above.
Although the general concept of the electromagnetic riveter has been the subject of a relatively large number of patents, such as U.S. Pat. No. 4,423,620 to HogenHout, et al, the basic technology is substantially as outlined above, using a voltage of 5000-10,000 volts, and current and magnetic force rise times of less than 250 microseconds, resulting in a force on the rivet of between several thousand and thirty thousand pounds, as required to accomplish the rivet formation.
However, even such an electromagnetic rivet forming system, with the advantages noted above, has significant operating disadvantages, including the danger of the very high voltages required and the resulting complexity, size, and high cost of the mechanical and electrical systems designed to handle such high voltages.
All of the elements of such a system must be capable of handling the high voltage levels. For instance, the switching devices which switch or "dump" the stored charge from the capacitors into the coil to create the fast rise time current pulse are ignitrons, which are specially designed to handle the high voltages involved. Long leads are required to connect the elements of the system, since the high voltage power supply is physically large and therefore cannot be positioned close to the workpiece. Further, the required power supply is quite expensive.
It has been discovered that internal cracks can develop in rivets formed by a high voltage electromagnetic riveting system. Such cracks, if not dangerous, are undesirable. They are caused by the extremely fast metal forming rates produced by the high voltage system. It is important to understand that the high voltage electromagnetic riveting system, like the pneumatic riveting gun, is a ballistic system, since the electrical pulse rise time is relatively short compared with the force rise time on the rivet. This means that the energy put into the system by the electrical pulse has been completed well before the work is completed, such that the operation of the system can be characterized by "hurling" the driver against the rivet. The fast metal forming rate can in fact be slowed by increasing the mass of the driver for a particular rivet, but such a mass increase decreases efficiency and is generally considered to be undesirable.
Hence, in summary, present electromagnetic riveting technology, while representing an advance in many respects over conventional pneumatic systems, is basically a large, expensive and generally inelegant apparatus, and requires significant safety precautions.
Accordingly, it is a basic object of the present invention to develop an electromagnetic actuator which is relatively safe to operate, relatively low in cost, and significantly smaller than existing electromagnetic actuator systems, and does not require a highly skilled operator.